As I run a few forums myself, I have disabled the ability to add images to signatures because it forces you to police them and when/if you do remove one, you get "Well, how come XYZ can have one like that and I can't??".
Furthermore, when an external image is used, they can very easily be changed to something that goes against the guidelines or, if they use a free image hosting site, you get those "Bandwidth Exceeded" images stuck on your forums.
Better to remove them.
I completely agree with this statement - having to keep a constant eye on signatures that may or may not break the rules - dealing with users that may take issues if their signatures is removed but others are allowed may end up costing Sapience and the moderators a lot of valuable time that they could be using gathering feedback about the game or communicating with the wider community - In an ideal world everyone would follow the rules, and maybe on a smaller forum it would be possible, but on such big boards I would maybe remove the option - It is Sapience call and it is not easy because some people will not like the signatures images option removed, you can never please everyone